A servo feed system has long been required in the electrical discharge machining field in which the known diode "OR" gate set up such as is shown in Matulaitis U.S. Pat. No. 2,783,411 issued on Feb. 26, 1957 for "Servo Feed for Multiple Electrodes" is improved in that the operating diodes array of the system are controlled in their conductivity to eliminate noise signals and more particularly to eliminate common mode voltages generated in the work cables. The present invention represents a substantial advancement over the prior art in this respect.
In the following specification when I refer to "electronic switch", I mean any electronic control device having three or more electrodes comprising at least two principal or power conducting electrodes acting to control current flow in a power circuit, the conductivity between the principal electrodes being controlled by a control electrode within the switch whereby the conductivity of the power circuit is controlled statically or electrically without movement of mechanical elements within the switch. Included within the definition are transistors in which "turn-on" is accomplished by a control voltage applied to the transistor control electrode and in which "turn off" is accomplished automatically in response to removal of that control voltage. Also included in the definition are devices of the gate type in which "turn-on" is accomplished by a control voltage applied to the control electrode, which control voltage may then be removed and in which "turn-off" is accomplished by the application of a subsequent control voltage to the control electrode.
An additional class of electronic switches, called "electronic trigger devices", falls within this definition and includes ignitrons, thyratrons, semi-conductor controlled rectifiers and the like. By "electronic trigger devices", I mean any electronic switch of the type which is triggered "on" at its control electrode by a voltage pulse and is turned "off" by a reverse voltage applied for a sufficient time period across its principal electrodes.
It will be understood that the present system will operate effectively, for example, when there is included in the EDM power supply a free running multivibrator or a pulse generator of the digitally controlled type, preferably including a counter and a digital control system such as the type shown and described in my U.S. Pat. No. 3,809,847 for "Method and Apparatus for Electrical Discharge Machining", issued on May 7, 1974. In that patent, there is shown a system whereby the desired "on" times and "off" times can be separately dialed and selected by the operator on decimal type thumb wheel switches which switches then furnish a continuing visual indication of the "on"-"off" times of the machining operation currently being carried on. Such a digital type pulse generator is particularly suitable for incorporation with the system of the present invention.